For example, Patent Document 1 discloses a laser printer as an electrophotographic image forming apparatus. The laser printer includes a housing and a process unit detachably disposed within the housing. The process unit includes a photosensitive member cartridge and a developing cartridge. The photosensitive member cartridge includes a photosensitive drum on a surface of which an electrostatic latent image is to be formed. A toner accommodation chamber is provided in the developing cartridge, and the developing cartridge includes a developing roller for supplying toner from the toner accommodation chamber to the photosensitive drum to visualize the electrostatic latent image on the photosensitive drum.
The developing cartridge is detachably attached to the photosensitive member cartridge. The photosensitive member cartridge includes a biasing unit for biasing the developing cartridge.
In a case where the developing cartridge is attached to the photosensitive member cartridge outside the housing, the biasing unit does not bias the developing cartridge, and therefore the developing roller of the developing cartridge is not pressed against the photosensitive drum of the photosensitive member cartridge although the developing roller may approach the photosensitive roller. When the photosensitive member cartridge having the developing cartridge attached thereto is installed in the housing as the process unit, the biasing unit of the photosensitive member cartridge biases the developing cartridge to press the developing roller against the photosensitive drum.
The presence of the biasing unit provides the following advantage: The developing cartridge is movable relative to the photosensitive member cartridge in a state in which the developing cartridge is attached to the photosensitive member cartridge, and hence, the developing roller is movable relative to the photosensitive drum to follow the movement of the developing cartridge relative to the photosensitive member cartridge. Accordingly, the developing roller can be appropriately moved depending on a surface condition of the photosensitive drum so as to follow the photosensitive drum and supply toner to the photosensitive drum in a smooth fashion. Compared with a case where a relative movement of the developing roller to the photosensitive drum is inhibited, the printer of this type can achieve a highly accurate visualization of an electrostatic latent image.    Patent Document 1: JP-A-2000-250378
For the printer of this type, it is conceivable to adopt an arrangement in which the toner accommodation chamber is separable from the developing cartridge. Since only the toner accommodation chamber, to which maintenance service is given highly frequently, can be replaced as a toner box, the utility of the printer can be increased.
However, because the developing cartridge is movable within the housing of the printer, the toner box separable from the developing cartridge has to be arranged movable within the housing to follow the movement of the developing cartridge in order to avoid toner leakage between the toner box and the developing cartridge. This makes the structure of the printer complicated. Further, because the toner box is substantially integral with the developing cartridge (and thus the process unit), it is difficult to attach and detach only the developing cartridge (process unit) to and from the housing separately from the toner box.